It is known to use ultrasound for improving the reaction speed when an exposed picture is to be developed and consequently is immersed in a bath of developer. The known apparatuses include one or more, usually many sound transducers mounted in such a manner in a tank with developer that the developer is caused to vibrate. A considerable power must be involved in order to provide the ultrasound in the known apparatuses with an essential influence on the reaction speed as the entire developer in the tank must be caused to vibrate.